As Phil mentions, I have started tests on tenderising lamb which are very much in their infancy. However, I can add that if you go along the route of using acids, such as citric acid from fruits (lemon, pineapple, papaya, etc.) and vinegars, you should not marinade the meat any longer than 3 hours, as they they will start to digest the meat rather than break down the collagen.
I have done a test on leg meat using Lemon juice, Garlic & Ginger Paste and Worcestershire Sauce mixed togther and thoroughly rubbed in. The result was ok but there is room for improvement. I will say that the Lemon was quite noticeable. I am formulating the next test, whilst awaiting for my delivery of Neck Fillet this week.
UF mentions a process that is important, which is 'stabbing' the meat, this has two functions: It enables you to massage the marinade deep within the meat but also helps to break down the meat fibres. I am currently using a fork to do this, but a filleting knife would be adequate.